1. Field of the Invention
The present invention relates to image forming apparatuses, such as copying machines, printers, facsimile machines, etc., and more particularly relates to an image forming apparatus that controls toner density of developer in a developing device to be constant by controlling toner supply to the developing device.
2. Discussion of the Background
In image forming apparatuses, such as copying machines, printers, facsimile machines, etc., an electrostatic latent image is formed on an image carrier, such as a photoconductor, and the latent image formed on the image carrier is developed into a visible toner image by supplying two-component developer, including toner, onto the latent image from a developing device. The developed toner image is then transferred to a transfer sheet. A developing bias voltage is applied to the developing device from a developing bias power source when the latent image is developed by the developing device, and toner is supplied to the developing device from a toner supplying device.
In such image forming apparatuses, because the toner density of two-component developer affects the density of an image to be developed with the developer, it is desirable to maintain the toner density of the two-component developer in the developing device constant. Therefore, a sensor, generally referred to as a "T" sensor, is provided in the developing device to detect the toner density of the two-component developer in the developing device. A detection result of the "T" sensor is compared with a criterion value for the toner density of the two-component developer. Then, the toner supply to the developing device from the toner supplying device is controlled according to a result of the comparison so that the toner density of the developer in the developing device is kept constant.
Further, the density of an image developed with developer significantly changes according to a change in the characteristics of the developer with the passage of time, the environmental conditions, such as temperature or humidity, or the developing conditions.
Therefore, generally, for obtaining toner images having a desired image density in a stable manner, a criterion image is formed on a surface of the image carrier with a predetermined criterion developing potential, which is a difference between a surface potential of a photoconductor as the image carrier and a developing bias potential. Then, the image density of the criterion image is detected by a reflective photo sensor, generally referred to as a "P" sensor. The criterion value for the toner density of two-component developer is then corrected according to a result of detecting the image density of the criterion image, and the toner supply to the developing device from the toner supplying device is controlled according to the corrected criterion value for the toner density.
Further, a method has been proposed in which a threshold value is provided for the criterion value for the toner density of two-component developer and the criterion value for the toner density is controlled such that the toner density of the two-component developer does not fall below a predetermined toner density.
More specifically, in such a method, each time an image forming operation is performed, the toner density of two-component developer in a developing device is detected by the "T" sensor and the quantity of toner to be supplied to the developing device is controlled according to a result of comparing an output value VT of the "T" sensor resulting from the detection and a criterion value VTref for the toner density. In addition, each time a predetermined number of images have been formed, a criterion image is formed on a photoconductor, the image density of the criterion image is detected by the "P" sensor, and a correcting amount .DELTA.VT for the criterion value VTref for the toner density is determined according to the output value of the "P" sensor resulting from the detection. A new criterion value VTref for the toner density is then determined based upon the correcting amount .DELTA.VT and the output value VT of the "T" sensor at the time of forming the criterion image. For example, the new criterion value VTref equals the previous criterion value VTref plus the correcting amount .DELTA.VT, i.e., VTref.sub.new =VTref.sub.previous +.DELTA.VT. When the new criterion value VTref.sub.new is smaller than a threshold value, the new criterion value VTref.sub.new is set as the criterion value for the toner density. When the new criterion value VTref.sub.new is larger than the threshold value, the threshold value is set as the criterion value for the toner density. The criterion value thus set is used for controlling the quantity of toner to be supplied to the developing device until the criterion image is formed next.
The above method has, however, a problem such that when an image forming operation is started after the image forming apparatus has been left unused for a long time under high temperature and high humidity, conditions for example during a summer vacation, an output value of the "T" sensor erroneously becomes higher than the output value before the apparatus had been left unused, for example by about 0.8 V, as illustrated in FIG. 1. If such an output value of the "T" sensor, which is erroneously higher than a normally detected value by about 0.8 V, is used as the output value of the "T" sensor, the toner density is determined as about 1.5 wt % lower than the actual toner density, as illustrated in FIG. 2. As a result, an excessive amount of toner is supplied and thereby the image density becomes excessively high. It has been known that the above erroneously higher output value VT of the "T" sensor is caused by a change in the density of the developer, which occurs when an image forming apparatus has been left unused for a long time.
An increase in the image density, which is caused by an error in the output value of the "T" sensor due to a change in the characteristics of developer, such as a change in the density of the developer as above, is too significant to be corrected by correcting the criterion value VTref for the toner density according to an output value of the "P" sensor such that the quantity of toner to be supplied is decreased. Even if the above increase in the image density due to the error in the output value of the "T" sensor is corrected for an initial stage of the image forming operations after a long period of non-use of the apparatus by correcting the criterion value VTref for the toner density such that the quantity of toner to be supplied is decreased, the output value VT of the "T" sensor returns to a normal value after the developer in the developing device has been stirred, for example for about 5 minutes, or after about 100 copies have been made, as illustrated in FIG. 1. Therefore, after the output value VT of the "T" sensor returns to the normal value, it occurs that the necessary quantity of toner is not supplied, and thereby the image density is decreased, because the criterion value VTref has been corrected such that the quantity of toner to be supplied is decreased and the same criterion value VTref is used until the criterion image is formed next and the criterion value VTref is again corrected according to an output value of the "P" sensor.